Prenatal development and chromosomal abnormalities
Critical periods of development
The most dangerous time for an unborn baby is the embryonic
period ( two weeks to three months fetal age) because this is when all of the basic organ systems are forming.
After the embryonic period, the unborn baby grows and matures,
but he has most of his basic biological equipment by this time.
Stages of prenatal development
an embryo – during the first two months.
a fetus- During the second trimester.
a baby - During the third trimester,
The embryo has three layers from which all
body organs develop.
Early formation of three layers of cells:
(1) the ectoderm, from which sense organs and nervous system will develop
(2) the mesoderm, from which
circulatory, skeletal and muscular systems will develop
(3) the endoderm, from which digestive and some glandular systems will
What are chromosomes?
Chromosomes are tiny string-like structures in cells of the body. They contain the estimated 20,000 to 25,000 human
gene pairs that determine traits like eye and hair color, as well as direct the growth and development of every part of our
physical and biochemical systems.
Each person normally has 23 pairs of chromosomes, or 46 in all. We normally inherit one chromosome per pair from our
mother and one from our father.
X-linked Recessive trait
Hemophilia A is a hereditary blood coagulation (clotting) disorder. It is caused by a deficient activity of plasma
protein factor VIII, which affects the clotting property of blood.
An inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis.
The group of diseases called muscular dystrophies includes many inherited disorders such as: Becker's muscular dystrophy
Ø Diabetes Mellitus
to metabolize carbohydrates and maintain proper glucose levels
Ø .Phenylketonuria (PKU)
A rare hereditary condition in which the amino acid phenylalanine is not properly metabolized; individuals with PKU
have a deficient enzyme required for the metabolism of the amino acid phenylalanine.
Ø Cystic Fibrosis
An inherited disease that affects the respiratory and digestive systems. Cystic fibrosis affects the mucus and sweat
glands of the body and is caused by a defective gene. Thick mucus is formed in the breathing passages in the lungs and this
predisposes the person to chronic lung infections. Many pancreatic enzymes involved in the breakdown and absorption of fats
in the intestine are absent, causing malabsorption (inadequate absorption of nutrients from the intestinal tract) and malnutrition.
Two Recessive Alleles in combination
Ø Sickle cell anemia
An inherited, chronic disease in which the red blood cells, normally disc-shaped, become crescent shaped. As a result, they function abnormally
and break down, causing recurrent painful episodes.
Ø Down syndrome
Down syndrome, in which a baby is born with an extra chromosome 21,
Sex Chromosomal abnormalities
Ø Turner’s syndrome (XO)
A sex chromosome abnormality: Girls with Turner syndrome have only one X chromosome, instead of the normal two.
Ø Triple X (XXX)
A sex chromosome abnormality: Girls with Triple X have only one extra X chromosome, instead of the normal two.
Ø Klinefelter’s syndrome (XXY)
males with Klinefelter syndrome have one extra copy of the X chromosome in each cell. Klinefelter syndrome is caused
by the presence of one or more extra copies of the X chromosome in a male's cells. Extra copies of genes on the X chromosome
interfere with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone.
Ø XYY Syndrome
Ø Fragile X
A change or mutation in a gene on the X chromosome causes the fragile X syndrome. the gene responsible for fragile
X syndrome is called the FMR1.